The field of the disclosure relates generally to multi-stage compressors, and more specifically to a multi-stage compressor with a variable speed drive.
Known multi-stage compressors provide two or more levels of compression, e.g., a two-stage compressor provides a high and low compression level. Many known heating ventilation and cooling (HVAC) systems, such as, for example, an air conditioner or a heat pump, utilize two-stage compressors to provide two levels of cooling capacity. One level, i.e., the high-capacity setting, provides cooling for hot, high-demand days. Another level, i.e., the low-capacity setting, provides cooling, for example, for milder days or other low-cooling demand periods of time. A typical installation utilizes the low-capacity setting 80% of the time, resulting in improved efficiency in operating the HVAC system. In such systems, the two-stage compressor operates for longer periods of time, produces less noise, and produces more even temperatures. Accordingly, multi-stage HVAC systems provide greater comfort and operate with greater efficiency.
A typical two-stage HVAC system operates at 100% capacity on the high-capacity setting and at about 66% capacity on the low-capacity setting. Such systems demonstrate an improved, i.e., higher, seasonal energy efficiency ratio (SEER) when operating at lower capacity. Efficiency improvements are gained in part by more efficient operation of the compressor, and also through operation of the indoor and outdoor fans at lower speeds. Typically, the system is more efficient at lower compressor capacity. Efficiency improvements are typically limited in this regard, in that the two-stage compressor cannot operate at a low enough capacity to match the cooling load or achieve the efficiencies of fully variable speed systems.